Oxygen detector for analysis of oxygen in gaseous streams including an internal humidifier

ABSTRACT

A galvanic oxygen detector comprising a metallic tubular housing having gas inlet and outlet means; a tubular and consumable anode concentrically mounted in intimate electrical contact with the inner surface of the housing; a tubular, porous and nonconductive electrolyte retentive matrix concentrically mounted in intimate contact with the inner surface of the anode; and a tubular and porous cathode concentrically mounted in intimate contact with the inner surface of the electrolyte matrix, the cathode defining a central chamber for confining the flow of gas that passes through the housing over the inner surface of the cathode. An external electrical circuit is connected between the cathode and the housing for measuring the current generated by the reduction of oxygen in the gas sample.

United States Patent 6/1966 Kordesch 204/195 [72] Inventors AmosLlnenberg Rehovot, Israel;

n m n om fie k I C BA 70 67 99 Ill 46 66 1 9 66 ll. 33

Herman S. Preiser, Ellicott City, Md. 839,803

FOREIGN PATENTS 21 Appl. No. 22 Filed July 2, 1969 [45] Patented Oct.26, 1971 Attorney-Finnegan, Henderson & Farabow ABSTRACT: A galvanic o[73] Assignee Hydrouautics, Inc.

Laurel, Md.

[54] OXYGEN DETECTOR FOR ANALYSIS OF xy'gen detector comprising ametalmm m kc 6 ea V m mm mm b .I .m o m m mmio gawnw .mefl n m Mm wanslu hm m m mm mmrm m rf d hm e mmdmw 9 N n A "4 0 mu m 2 U L C N l s m u sSEN- m wmnsm Ewm m 7 u N ENM hm YE X w M OWZU N U gas inlet and outletmeans; a tubular concentrically mounted in intimate the inner surface ofthe housin g; a tuyte retentive matrix ct with the inner surporouscathode concen- [51] B0lk3/00 [50] face of the anode; and a tubular andwith the inner surface of the electrolyte matrix, the cathode defining acentral chamber trically mounted in intimate contact asses through thehousing de. An external electrical for confining the flow of gas that pover the inner surface of the catho [56] References Cited UNITED STATESPATENTS 3,223,597 12/1965 Hersch 3,223,608

r .m n Se n8 x n mo n m d mu 0 .Mm oe mm v. Nb 0 m 3 n n We 8 b d e t ce n n o c .B .n u c .H c

324/29 measuring the current 324/29 the gas sample.

h c m e llll a ul l l l l ld Lni l V PATENTEBnm 25 I97! SHEET 2 OF 4 1INVI'INTORS AMOS LINENBERG HERMAN S. PREISER gm 7 l fla flasa/z ($26,55

ATTORNEYS PATENTEUnm 26 an SHEET Q 0F 4 l2 INVENTORS AMOS LINENBERGHERMAN S. PREI SER OXYGEN DETECTOR FOR ANALYSIS OF OXYGEN IN GASEQUSSTREAMS INCLUDING AN INTERNAL HUMEIFER This invention relates to oxygendetectors, and more particularly to new and improved galvanic cells fordetecting trace amounts of oxygen in gases.

In the past, various devices have been proposed for detecting andmeasuring the quantity of oxygen in a fluid by its heat conductivity,heat of combustion, electrolytic conductivity, paramagnetism, and manyothers.

It is known, for example, that gaseous oxygen can be produced in anelectrolytic cell by electrolysis of a suitable electrolyte, and thatthe amount of oxygen evolved is proportional to the amount of currentapplied to the cell. Similarly, it is known that oxygen can be reducedin a galvanic cell at one of the electrodes and generate a currentproportional to the amount of oxygen reduced.

Utilizing this latter principle, many and varied cell configurationshave been provided in which oxygen from a gas stream is distributed overan inert cathode of the cell while -a base metal anode undergoes anodicoxidation to produce a current proportional to the amount of oxygenreduced at the cathode. In a galvanic cell containing an aqueouselectrolyte, the following overall anodic oxidation, electrical-currentgenerating reaction occurs, where M represents a base metal anode:

1/201 M H2O M(OH)2 electrical energy The overall cell reaction isactually the result of subreactions that occur at both of the cell'selectrodes. In these subreactions, an oxygen containing gas supplied tothe cell reacts with the water of the electrolyte at the cathodeaccepting electrons generated at the anode and forming negativelycharged hydroxyl ions. At the anode, positively charged metal ions areformed in solution by giving up electrons. The base metal hydroxideproduced in the cell generally precipitates out of solution due to itslow solubility in the electrolyte.

The electrons generated at the anode travel to the cathode through anexternal circuit between the electrodes, and the amount of currentpassing through the circuit is proportional to the amount of oxygenreduced at the cathode. Measurement of the current generated, therefore,is indicative of the amount of oxygen in the gas.

While the anode is consumable and must eventually be replaced, anadvantage of a galvanic detector is that the cell immediately begins tooperate as soon as oxygen is introduced into the electrolyte without theneed for applying an external electromotive force. Galvanic cells,however, generally have an unfavorable cathode area to volume of carriergas ratio, requiring long gas residence times to insure completereduction of the oxygen in the gas. If complete reduction does notoccur, of course, the current produced in the cell will only be afraction of the current that should have been produced and will not be atrue measurement of the quantity of oxygen in the carrier gas.

Attempts to provide more intimate contact of the carrier gas with thecathode have included an electrode assembly comprising a single anodesandwiched between a porous electrolyte-cathode structure on eitherside, thus doubling the exposed cathode area in contact with the gas formore complete oxygen cathode diffusion and its subsequent reduction. Theelectrode assembly must be suspended within a suitable gasconfininghousing so that both surfaces of the cathode will be exposed to the gas.Such an assembly, however, not only complicates construction of the cellbut does not permit a smooth and uniform flow of gas through the cell,causing fluctuations in the amount of current produced and leading toerratic results, particularly at high gas flow rates. Further, thesuspended electrode assembly requires indirect electrical connectionsbetween the electrodes and an external measuring circuit, whichincreases the electrical resistance encountered and also leads toinaccurate results.

It is therefore a primary object of this invention to provide a new andimproved galvanic oxygen detector that can measure trace amounts ofoxygen in gases and provide stable and meaningful results at acceptablegas flow rates.

Yet another object of this invention is to provide a new and improvedelectrode assembly for a galvanic oxygen detector that provides a largeratio of cathode surface area to gas volume, that insures completereduction of the oxygen content of the gas, and that provides moredirect electrical connections between the external measuring circuit andthe electrodes to minimize current losses through electrical resistance.

A further object of this invention is to provide a galvanic oxygendetector having internal humidifying means for maintaining properhumidity in the detector.

Yet a further object of this invention is to provide a stronger, moredurable, and simplified oxygen detector that can be readily assembled,disassembled, and repaired and that is capable of operating under highpressures to improve oxygen cathode diffusion and its subsequentreduction.

Additional objects and advantages will be set forth in part in thedescription which follows, and in part will be obvious from thedescription or may be learned by practice of the invention, the objectsand advantages being realized and attained by means of the apparatus,methods, and improvements particularly pointed out in the appendedclaims.

To achieve the foregoing objects and in accordance with its purpose,this invention as embodied and broadly described provides apparatus fordetecting oxygen in a gas comprising:

a. a tubular housing having gas inlet and outlet means;

b. a tubular consumable anode incapable of evolving hydrogen upon beingcircuited with a cathode, said anode being concentrically mountedadjacent the inner surface of the housing;

c. a tubular, porous, and nonconductive electrolyteretentive matrixconcentrically mounted in intimate contact with the surface of the anodeopposite to the surface adjacent the housing;

d. a tubular and porous cathode concentrically mounted in intimatecontact with the surface of the electrolyte matrix opposite to thesurface in contact with the anode, said tubular cathode defining acentral passage for confining the flow of gas that passes through thehousing over the inner surface of the cathode; and

e. means for electrically connecting the anode and cathode to anexternal electrical circuit for measuring the current across said anodeand cathode.

In accordance with preferred embodiments of this invention, the housing,anode, electrolyte matrix, and the cathode are concentrically mountedcylinders; the housing is of metal and serves as the anode terminal ofthe cell; and the gas and electrical connections to the cell are allprovided in a single, readily removable and reusable cap for thehousing. In some embodiments, internal humidifying means are providedfor maintaining the proper humidity within the housing.

The accompanying drawings which are incorporated in and constitute apart of this specification illustrate presently preferred embodiments ofthe invention, and together with the description serve to explain theprinciples of the invention.

OF THE DRAWINGS:

FIG. I is a longitudinal cross section of a galvanic oxygen detectorembodying the present invention;

FIG. 2 is a broken-sectional view taken along the line 2-2 of FIG. 1;

FIG. 3 is a sectional view taken along the line 33 of FIG.

FIG. 4 is a longitudinal cross section of an alternative embodiment ofthis invention;

FIG. 5 is a broken-sectional view taken along the line 5-5 of FIG. 3;

FIG. 6 is a longitudinal cross section of yet another and simplifiedembodiment of this invention; the figure also illustrates schematicallya valving mechanism suitable for use with the detectors of thisinvention; and

FIG. 7 is a sectional view taken along the line 77 of FIG. 6.

Reference will now be made in detail to the present preferredembodiments of this invention, examples of which are illustrated in theaccompanying drawings.

As shown in FIG. 3., the galvanic oxygen detector of this inventionincludes a tubular housing, generally indicated as 10, having gas inletmeans 12, gas outlet means 14, and an axial chamber 15. Preferably,tubular housing 10 is a cylindrical metal tube constructed of stainlesssteel, such as 316 stainless, nickel, lnconel, and a variety ofnickel-chromium steels.

Housing i is capped at both ends with suitable pressuresealed fittingsto provide a pressure type housing while per mitting ready access to itsinterior. As best shown in FIG. 1, the pressure-sealed fitting on thebottom of housing includes a stainless steel, screw-type cap 16 thatfits over the end of the housing and threads into a retaining ring 17. Asimilar cap 18 and retaining ring 19 are provided on the top of housingit).

Gas inlet means 12 includes an inlet tube 24 that extends through anaxial aperture 22 in cap 16 and into the chamber of housing 10. inlettube 24 is of stainless steel and is fillet welded to the outer surfaceof cap 16 at 26.

Cap 16 is provided with a dished-out inner recess that is filled with aresinous material 28, such as an epoxy resin, to eliminate all voidsaround inlet tube 24. Preferably, a nonconductive sleeve 39 of Teflon orother suitable plastic material is fitted over the interior stub ofinlet tube 24 to insulate the metal tube from possible electricalcontact with any of the other elements located within housing 10.

In accordance with the invention, means are provided for diffusing theinlet gas and distributing the gas molecules over the entire cathodesurface. As embodied, the means comprises a microporous, sintered plug32, that is force-fitted into the inner open end of sleeve 30 to causethe inlet gas stream to break up into many minute streams as it passesthrough the plug and diffuse rapidly through chamber 15 in housing ll).Suitable materials for plug 32 include a sintered 316 stainless steelsheet having an average pore size of 10 4;, sintered polyethylene sheet,or any other commercial porous metallic or nonmetallic material.

In accordance with the invention, a tubular electrode assembly,generally indicated as 40, and preferably cylindrical in shape, isconcentrically mounted adjacent the inner surface of the lower portionsof housing 10.

As embodied, electrode assembly 40 includes a cylindrical anode 42containing a consumable metal capable of reducing oxygen in water butincapable of evolving hydrogen upon being circuited with a cathode. Achoice of metal for anode 42, therefore, is restricted to those having aposition sufficiently below hydrogen in the galvanic series (moreelectronegative) to provide an acceptable electromotive force and yetnot electrolyze water into its gaseous components of hydrogen and oxygenwhen connected to a suitable cathode. which would interfere with thefunction of the detector. Rather, the anode should dissolve in theelectrolyte of the cell to form cations upon being circuited with acathode. Suitable means for use in the construction of the anode includecadmium, lead, antimony, and bismuth, and the like. Exemplary of anunsuitable metal for the anode is zinc because of its higherelectrochemical activity which would cause it to reduce water tohydrogen when connected to a noble cathode such as graphite or carbon.

Preferably, anode 42 is a reticulated porous substrate impregnated withone of the above metals. Exemplary of a preferred anode is acommercially available cadmium impregnated nickel screen. The screen isrolled into a cylinder with its outer diameter slightly larger than theinner diameter of housing l0 so that it can be force-fitted into thehousing and be in electrical contact with the inner surface of the metalhousing. To insure a good electrical connection between housing 10 andanode 42, the ends 44 of the anode are dipped in a l-normal hydrochloricacid solution to dissolve the cadmium from the nickel screen. Theexposed nickel screen ends are then tack welded to the inner surface ofthe metal housing.

Electrode assembly 40 also includes a tubular, porous, and nonconductiveelectrolyte retentive matrix 46 that is concentrically mounted inintimate contact with the inner surface of anode 42. As embodied,electrolyte matrix 46 is cylindrical and is tightly fitted against theinner surface of anode cylinder 42 and extends slightly beyond its ends44. Suitable nonconductive porous materials for use as an electrolytematrix include finely woven cloths of polypropylene, Teflon,polyethylene, preformed sintered cylinders of similar plastic materials,and the like. A preferred material for use as electrolyte matrix 46 is anylon felt, available from the Pellon Corporation. The nylon felt isrolled into a spirally wrapped, fourlayered cylinder and force-fittedinto the anode cylinder.

As further described below in connection with the operation of thepresent device, matrix 46 is impregnated with an aqueous electrolytesolution. Satisfactory electrolytes include alkali metal hydroxides,such as potassium or sodium hydroxide. A preferred electrolyte for usein the present device is a 20 percent aqueous solution of potassiumhydroxide.

Finally, electrode assembly 40 includes a tubular, porous cathode 48concentrically mounted in intimate contact with the inner surface ofelectrolyte matrix 46. Preferably, cathode 48 is also cylindrical anddefines a central passage 50 for confining the entire gas flow over theexposed inner surface of the cathode, thereby permitting the gas todiffuse through the cathode and establish a gas interface with theelectrolyte in matrix 46. As embodied, cathode 48 is a reticulatedporous cylinder whose length is slightly less than electrolyte matrix46. l

Cathode 48 can be formed of a porous layer of a variety of inert metals,such as silver, gold, carbon, or the precious metals of Group 8 of thePeriodic Table (i.e., having an atomic number of at least 44), such asplatinum. Preferably, cathode 48 is a graphite impregnated woven satincloth. The cloth is spirally wrapped into a four-layered cylinder thatfits tightly against the inner surface of electrolyte matrix 46.

As shown in FIG. 1, a split U-ring 51, preferably of silver, ispressure-crimped to the upper end of cylindrical cathode 48. When arelatively rigid, porous material is used for electrolyte retentivematrix 46, a recess 54, as shown in FIG. 1, can be provided around theinner top of the matrix to provide clearance for crimping ring 51.

A plurality of retaining rings 56, suitably of nonconductive plasticmaterials, such as polyvinylchloride, polyethylene, Teflon, and thelike, are spaced axially along the inner surface of cathode 48 to holdthe cathode in intimate contact with electrolyte matrix 46.

An electrical lead 52, preferably of silver, is swagged at one end toring 51 and at the other end to a gastight, ceramic or glass insulatedelectrical feed-through connector 60 that passes through an epoxy resinseal 62 in the side of housing 10. Connector 60 thus serves as thecathode terminal for the cell. Surrounding connector 60 and welded tothe outer surface of housing 10 is a threaded fitting 64 that serves asthe anode ter' minal for the cell since the metal housing is inelectrical contact with anode 42.

A shorting resistor 66, generally from about 10 and ohms, is connectedbetween cathode connector 60 and housing 10 to short out excessivecurrent loads on an external measuring circuit (not shown).

Gas outlet means 14 for exhausting carrier gas after it has passedthrough housing 10 includes an outlet tube 67 that extends through topcap 18 and communicates with the housings chamber 15. Preferably, gasoutlet tube 67 is horizontal, as shown in FIG. I, so that it does notinterfere with the servicing of the humidifying means, as more fullydescribed below.

In accordance with the invention, internal humidifying means areprovided for maintaining the proper humidityin the cell, and moreparticularly to prevent a drying out of the electrolyte. As embodied,and as shown in FIG. 1, the humidifying means, generally indicated at68, includes an external transparent tube 69 that extends axiallythrough an opening 70 in the top of cap 18 and into housing chamber l5.Tube 69 serves as a sight glass and is sealed into position by an epoxyseal 72 and an O-ring 74, retained in corresponding recesses in cap 18.A very fine porous glass tube 76 is fitted to the internal end of sightglass 69 and extends axially through housing chamber 15 into centralpassage 50, terminating a slight distance above plug 32 in gas inlettube 24. The external end of sight glass tube 69 is fitted with aretaining ring 84) and a corresponding screw-type cap 82 containing apierceable elastometric seal 84. Seal 843 can be of butyl rubber or anyother soft inert material that permits periodic injection of ahumidifying solution into porous tube 76.

A thin vent tube 86 of polyethylene or other inert plastic is locatedaxially within porous glass tube 76 and is held in place by an annularplug 88 in the bottom of tube 76. Vent tube 86 is open at both ends, thetop being located adjacent plug 84 and the bottom adjacent the diffuserplug 32 in gas inlet tube 24.

As shown in the drawings, the axial construction of humidifier 68, inaccordance with this invention, not only provides an internal means formaintaining proper humidity control in the cell, but it further confinesthe flow of gas through central passage 50, thereby increasing the ratioof cathode surface area to gas volume and insuring more completereduction of the oxygen content of the gas.

in assembly, electrode assembly 40 is placed within housing in themanner described above and the housing, without end caps 16 or 18, isplaced in a suitable closed container and the container is evacuated.The container is then slowly filled with an. aqueous electrolytesolution, such as a 20 percent potassium hydroxide, to impregnateelectrolyte matrix 46 with the electrolyte. The vacuum in the containeris then released by admitting air and the entire procedure is repeatedto insure complete impregnation of the electrolyte matrix. The cellshould then be tested in air by a suitable meter shorted through aresistor to produce a low impedance of about l0 to 100 ohms. A currentof about I00 #8. is indicative of a good cell. If a lower output isobtained, impregnation steps should be repeated.

An insulating washer 90 is placed in the lower end of housing 10 toprevent cap 16 from shorting the ends of the electrodes, and the cap isthen assembled into retaining ring 17.

Porous glass tube 76 of humidifier 68 is filled with a 20 percentpotassium hydroxide solution (oxygen free) being careful to fill aroundand up to just below the level of vent tube 86. Upper cap 18 containinghumidifier 68 is then assembled into retaining ring 19 to provide agastight seal at the upper end of housing 10.

in operation, and as best shown by reference to the drawings, an oxygencontaining gas is fed to housing 10 through inlet tube 24. The gas isdispersed through porous plug 32 and passes into central passage 50 andinto contact with the inner surface of porous cathode 48. The flow ofgas through chamber 50 and over cathode 48 is confined to a thin, widesheet by the outer surface of axial tube 76 of humidifying means 68,thus increasing oxygen cathode contact and its subsequent reduction.

As the oxygen containing gas flows upwardly through passage 50 in thedirection of arrows 96 under the pressure of the inlet gas, it diffusesthrough porous cathode 48 and comes into contact with the electrolyte,which is dispersed throughout matrix 46, forming a gas-electrolyteinterface within cathode 48. The oxygen content of the gas is reduced atthe cathode by reaction with the water of the electrolyte, forminghydroxyl ions. The hydroxyl ions are transported by the electrolyte toanode 42 where they oxidize the base metal of the anode and produce theelectrical output of the detector.

External measuring means, not shown, are provided by measuring theelectrical output. Thus, the amount of current flowing in the externalcircuit is proportional to the amount of oxygen consumed at cathode 48.and, therefore, can be used to measure the oxygen content of the gas.

Since accurate results are dependent upon complete reduction of all thegases oxygen content, a relatively large cathode surface area to gasvolume is provided in the device of this invention. Further, the inletgas can be pressurized to insure more complete oxygen cathode diffusionand its subsequent reduction.

The oxygen depleted gas passes out of central passage 50 into the upperend of housing chamber 15 and out of the housing through outlet tube 67.

If a dry gas is passed through the cell and absorbs water from theelectrolyte, the difference in vapor pressures thereby establishedbetween the electrolyte and the saturated condition of humidifier 63causes porous tube 76 to sweat water vapor and restore the properhumidity in the cell. Thus, the fine pores of tube 76, which permit onlythe penetration of water vapor and not water molecules, provides aninternal means for maintaining humidity control without flooding theelectrolyte with excess moisture.

Vent tube 36 is provided to equalize the pressures between housingchamber 50 and the interior of tube 76 and thereby permits the watervapor to pass through the pores of the tube. Sight glass 69 indicateswhen refilling of humidifier tube 76 is required. To refill, ahypodermic syringe is pierced through plug 84 and the necessaryquantities of potassium hydroxide solution are injected into tube 76around vent tube 86.

While it is preferred to use an aqueous electrolyte solution in thehumidifier column, so that the vapor pressures of the electrolyte andthe humidifier are the same, oxygen-free water can also be used in lieuof the electrolyte solution.

In accordance with an alternative embodiment of this invention, and asshown in FIG. 4, an impervious ring 92 is wedged between the outersurface of porous glass tube 76 and the top of cathode 48. Ring 92blocks the outlet end of central passage 50 to force all of the gas topass through porous cathode 48 and thereby insures a more completereduction of its oxygen content.

As further shown in FIG. 4, capillary rings 94 can be provided betweenthe outer surface of porous tube 76 and cathode 48 to assist in thetransfer of water from the humidifier to electrolyte matrix 48 toprevent a drying out of the matrix.

In accordance with a simplified embodiment of this invention, and asshown in FIGS. 6 and 7, housing 10 comprises a deep drawn cylindricalmetal tube having a closed end and an open end 102. A cap 104 fits overthe open end 102 of the housing and includes an O-ring seal I06 thatsnap fits into channel 108 in the outer side walls of housing 10 toprovide a sealed pressure type housing while permitting access to itsinterior.

All gas and electrical connections are provided in cap 104 to simplifyconstruction, assembly, and repair of the detector. Hence, as embodiedand as shown in FIG. 6, a gas inlet tube 110 and a gas outlet tube 112are both secured to and extend through cap 104 into the interior ofhousing 10. The inner recess of cap 104 is filled with a sealing andinsulating plug 114 to prevent leakage of gas around the tubes 110 and112 and to prevent the cap from shorting the ends of the electrodes. Asimilar insulating plug 116 is inserted into the closed end 1% of tubeit) to prevent shorting the other ends of the electrodes.

The tubular electrode assembly 40 of this embodiment is substantiallysimilar to the assembly described in the previous embodiments andincludes a cylindrical anode 42, a cylindrical and nonconductiveelectrolyte-retentive matrix 46, and a cylindrical porous cathode 48that defines a central passage 50 for confining the flow of gas throughhousing 10.

An electrical lead 118, preferably of silver, is threaded through porouscathode 48 and passes out through plug 114 in cap 104 where it isconnected to a terminal connector 60 that serves as the cathode terminalfor the cell. Fitting 64, welded to cap 104, serves as the anodeterminal as in the previous embodiments.

In this embodiment of the invention, an impervious cylindrical core 120extends substantially through the entire length of central passage 50 ina manner similar to humidifier tube 76.

Core 120 is closely spaced from the inner surface surface of porouscathode 48 and thus confines the flow of gas in passage 50 to insuremore complete reduction of its oxygen content. Preferably, the outersurface of core 120 is fluted to center it within the cathode andprovide a plurality of passages for the flow of the gas.

In accordance with this invention, an axial conduit 122 is provided incore 120 for exhausting the gas after it has passed over cathode 48. Asshown in FIG. 6, conduit 122 communicates with passage 50 through ports124 in the upper end of core 120 and with gas outlet tube 112 at thelower end of core 120.

Operation of the embodiment of FIGS. 6 and 7 is substantially similar tothe previous embodiments, gas entering housing 10 through inlet tube 110and then passing upwardly through passage 50 where it contacts the innersurface of cathode 48 and forms an interface with the electrolyte inmatrix 46. The oxygen depleted gas passes out of passage 50 and intoconduit 122 in core 120 and then out of.the housing through gas outlettube 112.

The oxygen detector of this embodiment is contemplated for use with anexternal humidifier so that it can be kept compact and as simplified aspossible, but it will be apparent that the device can easily be modifiedto accept internal humidifying means, if desired, without departing fromthe: scope of this invention.

Preferably, and as schematically illustrated in connection with theembodiment of FIG. 6, a four-way valve I30 is used to control the flowof gas to the oxygen detectors. The valve, of course, can be used withany of the embodiments of this invention, but for convenience will bedescribed as it relates to the embodiment of FIG. 6.

Valve 130 schematically includes a valve block I32, a sample gas inlet138, a sample gas outlet 140, a vent 142, and passages 134 and 136 forconnecting the valves inlet and outlet to the inlet and outlet means 12and 14, respectively, of the oxygen detector. Thus, with valve block-132in the extreme left or OFF position, the gas bypasses the detector andthe inlet and outlet means of the cell are closed by the valve block toprevent the cell from being contaminated by ambient air.

To take a sample of the gas for analysis, valve block 132 is simplymoved to the extreme right or ON position, as shown in F IG. 6, so thatthe gas now passes through the cell where its oxygen content is measuredas a function of the generated current. After a meaningful reading isobtained, the detector is then disconnected by moving the valve back tothe "OFF" position to prevent unnecessary waste of the consumable metalanode.

Thus, it will be apparent from the foregoing description that thisinvention provides a unique concentric construction for the electrodeassembly of a galvanic oxygen detector that provides a high cathodesurface area to gas volume ratio for maximum galvanic efficiency and asmooth gas flow for stable current outputs, leading to more constant andreliable results. Further, the concentric construction of the electrodeassembly, and particularly in combination with the compact constructionof the housing as shown in H6. 6, permits the device to be readilyassembled and disassembled for inspection, maintenance, replacement ofparts, and repair.

By providing an outer metallic housing for the detector in accordancewith the invention, the housing can be used directly as the externalelectrical connection for the anode of the cell to minimize currentlosses through electrical resistance and, in combination with thegastight end seals, provides a pressurizable container for improvingoxygen cathode diffusion.

In addition, the unique internal humidifier of this invention not onlyprevents a drying out of the electrolyte by maintaining a constanthumidity within the cell, but its axial location confines the flow ofgas, without disturbing its flow characteristics, to increase thecathode surface area to gas volume ratio and insure more rapid andcomplete oxygen cathode diffusion.

The unique construction of the internal humidifier of this inventionfurther permits monitoring and replenishing of the humidifying solutionwithout disassembly of the detector device.

The invention in its broader aspects is not limited to the specificdetails shown and described and departures may be made from such detailswithout departing from the principles of the invention and withoutsacrificing its chief advantages.

What is claimed is:

1. Apparatus for detecting oxygen in a gas comprising:

a. a tubular housing having gas inlet and outlet means;

b. a tubular consumable anode incapable of evolving hydrogen upon beingcircuited with a cathode, said anode being concentrically mountedadjacent the inner surface of the housing;

c. a tubular porous and nonconductive electrolyte-retentive matrixconcentrically mounted in intimate contact with the surface of the anodeopposite to the surface adjacent the housing;

d. a tubular and porous reticulated cathode concentrically mounted inintimate contact with the surface of the electrolyte matrix opposite tothe surface in contact with the anode, said tubular cathode defining acentral passage for confining the gas flow through the housing over theinner surface of the cathode;

e. a core impervious to the passage of gas located within the centralpassage in the tubular cathode to confine the flow of gas passing overthe cathode into a thin tubular sheet; and

f. means for electrically connecting the anode and the cathode to anexternal electrical circuit for measuring the current across said anodeand cathode.

2. The apparatus of claim I, in which the tubular housing is metallicand in electrical contact with the anode.

3. The apparatus of claim 2, in which the tubular housing is ofstainless steel.

4. The apparatus of claim 1, in which the housing, the anode, theelectrolyte matrix, and the cathode are concentrically mountedcylinders.

5. The apparatus of claim 1, in which the anode contains an oxidizablemetal selected from the group consisting of lead, antimony, bismuth,cadmium, and mixtures thereof.

6. The apparatus of claim 5, in which the anode comprises a cadmiumimpregnated nickel screen.

7. The apparatus of claim 1, in which the cathode contains an inertelectrically conductive material selected from the group consisting ofsilver, gold, carbon, and elements of Group 8 of the Periodic Tablehaving an atomic number of at least 44.

8. The apparatus of claim I, in which the cathode is a graphite-impregnated cloth.

9. The apparatus of claim I, in which the electrolyte matrix is a thinsheet of porous polymeric material.

10. The apparatus of claim 9, in which the matrix is a nylon felt.

11. The apparatus of claim I, in which the electrolyte is an aqueoussolution of an alkali metal hydroxide.

12. The apparatus of claim ll, in which the electrolyte is a 20 percentaqueous solution of potassium hydroxide.

123. The apparatus of claim 1, in which the gas inlet and outlet meansare located at opposite ends of the tubular housing.

14. The apparatus of claim 1, wherein the gas inlet and outlet means arelocated at the same end of the housing, and the impervious core extendsthrough the central passage in the tubular cathode from one end of thehousing to the other and has an axial conduit communicating with thecentral passage opposite from said end of the housing so that the gasenters and passes over the cathode in one direction and then flows backthrough the axial conduit in the opposite direction to be exhaustedthrough the outlet means.

15. The apparatus of claim 14, in which the tubular housing is metallicand in electrical contact with the anode.

16. The apparatus of claim 14, in which the housing is cupshaped and theanode, matrix, and cathode are concentrically mounted cylinders withinthe housing, said gas inlet and outlet means being located in aremovable cap for the housing.

17. The apparatus of claim 1, including internal humidifying means formaintaining the proper humidity within the tubular housing andpreventing a drying out of the electrolyte.

18. The apparatus of claim 17, in which the impervious core is thehumidifying means and comprises a fine porous glass tube for confining aquantity of an aqueous solution, the tube permitting water vapor to passthrough the pores of the tube and into the housing when necessary tomaintain proper humidity in the housing.

19. The apparatus of claim 18, including means permitting refilling ofthe porous glass tube with the aqueous solution externally of thehousing.

20. The apparatus of claim 1 including an impervious ring wedged betweenthe outer surface of the impervious core and the top of the cathode toforce all of the gas to pass through the cathode at its top.

21. The apparatus of claim 16 wherein the outer surface of the core isfluted to center it within the cathode and provide a plurality ofpassages for the flow of the gas.

22. The apparatus of claim 18 including at least one capilla ry ringbetween the outer surface of the porous glass tube and the cathode toassist in the transfer of water from the humidifier to the electrolytematrix to prevent a drying out of the matrix.

23. Apparatus for detecting oxygen in a gas comprising:

a. a tubular metallic, electrically conductive housing having gas inletand outlet means;

b. a tubular consumable anode incapable of evolving hydrogen upon beingcircuited with a cathode, said anode being mounted in intimateelectrical contact with an inside surface of the housing;

c. a tubular porous and nonconductive electrolyte-retentive matrixmounted in intimate contact with the surface of the anode opposite tothe surface adjacent the housing;

d. a tubular porous reticulated cathode having a central passage exposedto the flow of gas through the housing and being mounted in intimatecontact with the surface of the electrolyte matrix opposite the surfaceadjacent the anode;

e. means for electrically connecting the cathode and the housing, as theanode conductor, to an external electrical circuit for measuring thecurrent across said anode and cathode; and

f. an impervious core extending through the central passage in thetubular cathode to confine the flow of gas passing over the cathode intoa thin tubular sheet.

24. An internal humidifier for an oxygen detector cell containing anaqueous electrolyte-impregnated matrix, said humidifier comprising afine porous glass tube extending into the interior of the cell and incontact with the gas passing through the cell for confining a quantityof an aqueous solution and permitting water vapor to pass through thepores of the tube and into the matrix when necessary to maintain properhumidity in the cell.

i t i

2. The apparatus of claim 1, in which the tubular housing is metallicand in electrical contact with the anode.
 3. The apparatus of claim 2,in which the tubular housing is of stainless steel.
 4. The apparatus ofclaim 1, in which the housing, the anode, the electrolyte matrix, andthe cathode are concentrically mounted cylinders.
 5. The apparatus ofclaim 1, in which the anode contains an oxidizable metal selected fromthe group consisting of lead, antimony, bismuth, cadmium, and mixturesthereof.
 6. The apparatus of claim 5, in which the anode comprises acadmium impregnated nickel screen.
 7. The apparatus of claim 1, in whichthe cathode contains an inert electrically conductive material selectedfrom the group consisting of silver, gold, carbon, and elements of Group8 of the Periodic Table having an atomic number of at least
 44. 8. Theapparatus of claim 1, in which the cathode is a graphite-impregnatedcloth.
 9. The apparatus of claim 1, in which the electrolyte matrix is athin sheet of porous polymeric material.
 10. The apparatus of claim 9,in which the matrix is a nylon felt.
 11. The apparatus of claim 1, inwhich the electrolyte is an aqueous solution of an alkali metalhydroxide.
 12. The apparatus of claim 11, in which the electrolyte is a20 percent aqueous solution of potassium hydroxide.
 13. The apparatus ofclaim 1, in which the gas inlet and outlet means are located at oppositeends of the tubular housing.
 14. The apparatus of claim 1, wherein thegas inlet and outlet means are located at the same end of the housing,and the impervious core extends through the central passage in thetubular cathode from one end of the housing to the other and has anaxial conduit communicating with the central passage opposite from saidend of the housing so that the gas enters and passes over the cathode inone direction and then flows back through the axial conduit in theopposite direction to be exhausted through the outlet means.
 15. Theapparatus of claim 14, in which the tubular housing is metallic and inelectrical contact with the anode.
 16. The apparatus of claim 14, inwhich the housing is cup-shaped and the anode, matrix, and cathode areconcentrically mounted cylinders within the housing, said gas inlet andoutlet means being located in a removable cap for the housing.
 17. Theapparatus of claim 1, including internal humidifying means formaintaining the proper humidity within the tubular housing andpreventing a drying out of the electrolyte.
 18. The apparatus of claim17, in which the impervious core is the humidifying means and comprisesa fine porous glass tube for confining a quantity of an aqueoussolution, the tube permitting water vapor to pass through the pores ofthe tube and into the housing when necessary to maintain proper humidityin the housing.
 19. The apparatus of claim 18, including meanspermitting refilling of the porous glass tube with the aqueous solutionexternally of the housing.
 20. The apparatus of claim 1 including animpervious ring wedged between the outer surface of the impervious coreand the top of the cathode to force all of the gas to pass through thecathode at its top.
 21. The apparatus of claim 16 wherein the outersurface of the core is fluted to center it within the cathode andprovide a plurality of passages for the flow of the gas.
 22. Theapparatus of claim 18 including at least one capillary ring between theouter surface of the porous glass tube and the cathode to assist in thetransfer of water from the humidifier to the electrolyte matrix toprevent a drying out of the matrix.
 23. Apparatus for detecting oxygenin a gas comprising: a. a tubular metallic, electrically conductivehousing having gas inlet and outlet means; b. a tubular consumable anodeincapable of evolving hydrogen upon being circuited with a cathode, saidanode being mounted in intimate electrical contact with an insidesurface of the housing; c. a tubular porous and nonconductiveelectrolyte-retentive matrix mounted in intimate contact with thesurface of the anode opposite to the surface adjacent the housing; d. atubular porous reticulated cathode having a central passage exposed tothe flow of gas through the housing and being mounted in intimatecontact with the surface of the electrolyte matrix opposite the surfaceadjacent the anode; e. means for electrically connecting the cathode andthe housing, as the anode conductor, to an external electrical circuitfor measuring the current across said anode and cathode; and f. animpervious core extending through the central passage in the tubularcathode to confine the flow of gas passing over the cathode into a thintubular sheet.
 24. An internal humidifier for an oxygen detector cellcontaining an aqueous electrolyte-impregnated matrix, said humidifiercomprising a fine porous glass tube extending into the interior of thecell and in contact with the gas passing through the cell for confininga quantity of an aqueous solution and permitting water vapor to passthrough the pores of the tube and into the matrix when necessary tomaintain proper humidity in the cell.